3B2v7:51c ED:mamatha=brr GML4:3:1 MAGMA : 8449 Prod:Type:com pp:124ðcol:fig::NILÞ PAGN: mva SCAN: radhika ARTICLE IN PRESS 1 3 Journal of Magnetism and Magnetic Materials ] (]]]]) ]]]­]]] 5 7 Effect of interface roughness on magnetic multilayers of 9 Fe/Tb and Fe/Cr 11 Amitesh Paul* 13 Institut f.ur Festk.orperforschung, Forschungszentrum J.ulich GmbH, D-52425 J.ulich, Germany 15 17 Abstract 19 The effect of systematic variation in the correlated interface roughness on perpendicular magnetic anisotropy (PMA) and giant magnetoresistance (GMR) has been studied in Fe/Tb and Fe/Cr multilayer systems, respectively. Multilayers 21 for each system were deposited simultaneously on a set of float glass substrates pretreated with varying rms surface roughness. In both the systems the amount of intermixing at the interfaces and other morphological parameters are 23 found similar, thus allowing one to separate out the effect of interface roughness only. X-ray reflectivity, diffuse scattering, conversion electron M.obbauer spectroscopy and superconducting quantum interference device magneto- 25 metry are used to characterise the systems. With the increase in s; the PMA in Fe/Tb as well as the GMR in Fe/Cr shows a small decrease. The observed effects are mainly due to the changes in the correlated part of the roughness of the 27 multilayers, while the uncorrelated part of the s of different multilayers are expected to remain similar. r 2001 Published by Elsevier Science B.V. 29 PACS: 75.70 i; 68.35Ct 31 33 1. Introduction [2]. Therefore, in the present study, MLs for both the 35 systems of Fe/Tb and Fe/Cr are deposited on substrates 57 Magnetic multilayers (MLs) showing properties like pretreated with varying surface roughnesses. It has been 37 perpendicular magnetic anisotropy (PMA) in systems seen that except for the interface roughnesses, other 59 like Fe/Tb MLs or giant magnetoresistance (GMR) in microstructural features of the ML like grain size, 39 Fe/Cr MLs are significantly affected by their interfacial coherence length, grain texture, intermixing at the 61 structures [1,2]. interface, internal stresses etc. are similar, thus allowing 41 Earlier studies on Fe/Tb MLs, to see the effect of one to selectively study the effect of interface structure 63 interfacial modifications on PMA, were mainly done by (varied systematically) on PMA in Fe/Tb and on GMR 43 post-deposition treatments like thermal annealing [3] or in Fe/Cr MLs. 65 ion irradiation [1]. However, the induced effects include 45 changes in geometrical roughness as well as intermixing/ 67 demixing at the interface. Therefore, it has not been 47 possible to separate the effects of interface roughness (s) 2. Experimental details 69 from that of intermixing/demixing [1]. Experimental 49 results on the 71 51 73 53 UNCORRECTED PROOF effect of s on GMR are also conflicting. It Substrates with varying surface roughness were has been seen that depending upon the ratio of the spin prepared in two sets by etching the float glass (FG) asymmetry for the interface and bulk scattering and the substrates in dilute HF for different periods of time. various techniques used to modify the interfaces there is Set1: Eight substrates with increasing etching times of 0, either an increase or decrease of GMR with roughness 15, 30, 60, 90, 120, 150 and 180 s, designated as S1­S8, 75 respectively, were taken. The multilayer consisted of 20 55 *Fax: +49-2461-61-4443. bilayers of composition 3.0 nm Fe/2.0 nm Tb, were 77 E-mail address: a.paul@fz-juelich.de (A. Paul). deposited on FG substrates. Set2: A set of substrates 0304-8853/01/$ - see front matter r 2001 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 9 1 2 - X MAGMA : 8449 ARTICLE IN PRESS 2 A. Paul / Journal of Magnetism and Magnetic Materials ] (]]]]) ]]]­]]] 1 were prepared for 14 different etching times which are show distinct oscillatory variation of the substrate 57 numbered as S1­ S14 and which show similar results as roughness with increasing etching time. The fitting of 3 that of Set1. In this set MLs consisted of the following the reflectivity and rocking curve patterns for the 59 deposition sequence: substrate/Cr (10.0 nm)/[Fe substrates, done by simulations following theories 5 (3.0 nm)/Cr (1.2 nm)] 20/Fe (5.0 nm). Deposition con- [4,7], gives the value of s; x (lateral correlation length) 61 ditions are similar as reported in Refs. [2,3] B450750 nm and h (Hurst parameter measuring 7 A powder X-ray diffractometer model D5000 of jaggedness)=0.270.1. 63 Siemens with Cu Ka radiation was used to measure CEMS spectra for the MLs are fitted with two 9 the specular (XRR) and diffuse scattering geometry subspectra: one sharp (a-Fe) and other broad one 65 (XDS) [4]. 57Fe conversion electron M.obbauer Spectro- corresponding to the Fe atoms at the interface. 11 scopy (CEMS) was used to get information about the Intermixed layer thickness inferred from the area under 67 intermixing at the interface and the PMA at room the sharp sextet shows no significant increase 13 temperature using a gas flowing (95% He, 5% CH4) (1.070.3 nm) with etching time and is essentially being 69 proportional counter. The spectral profiles were ana- used as an input parameter in XRR curve fitting. The 15 lysed by means of the NORMOS code developed by probability of hyperfine field distribution PðBhfÞ and the 71 Brand [5]. The magnetic texture of the sample is revealed average /BhfSðTÞ was also similar. Fig. 1 shows the 17 by the intensity of the 2nd and 5th peaks relative to the specular (subtracted off the off-specular) X-ray patterns 73 inner ones of the M.obbauer spectrum. RF SQUID for the specimens. The patterns clearly show the first- 19 measurements were done at 4.2 K (QUANTUM DE- order Bragg peak due to ML periodicity and a distinct 75 SIGN model MPMSR2) with the field being in the film oscillatory variation of the s with increasing etching 21 plane and the ratio of magnetic remanence Mr and the time. The values of the substrate (ss) and the interface 77 magnetic saturation Ms was used to infer the extent of roughness (si) for the MLs with sample nos. S1, S2, S4, 23 antiferromagnetic coupling fraction (AFF) given by S5 and S6 are also given with the figure. The similar 79 (1 Mr=Ms) [6]. increment in s2 (s2i2s2) from substrate to the ML 25 interfaces signifies that the change in roughness by 81 substrate roughness variation is only affecting the 27 correlated part of the roughness of the MLs while the 83 3. Results and discussion uncorrelated part of the roughness remains unaffected. 29 The parameters could not be extracted for the sample 85 3.1. Fe/Tb MLs nos. S3, S7 and S8 as the intensity of the off-specular 31 scans is comparable to that of the specular scans, which 87 The XRR pattern of the float glass substrates of Set1 also signifies that the peak at the first Bragg position for 33 and Set2 subjected to different etching time (see Ref. [2]) the specimens with higher substrate roughness is arising 89 35 91 37 experimental 93 S 1 theoretical s= 0.6 5 ± 0.0 5 n m 39 = 1.7 0 ± 0.0 5 n m i 95 S1 = 0 .9 4 5 0 S 2 41 = 1 .2 0 n m s 97 = 1 .9 0 n m i 43 ~ 3 0 0 n m c 99 S 4 (± 5 0 n m ) 45 = 1 .9 5 n m s 101 = 2 .4 5 n m i = 0 .9 7 5 0 S 5 47 = 1 .1 5 n m s = 1 .7 0 n m 103 Log intensity (arb.units) i ~ 5 0 n m u S 6 49 = 1 .2 5 n m s (± 1 0 n m ) 105 = 1 .9 5 n m i -0 .8 0 .0 0 .8 51 0 .2 0 .4 0 .6 0 .8 1 .0 1 .2 - (deg.) 107 Incident angle (deg) 53 Fig. 1. XRR UNCORRECTED PROOF 109 scans of [Fe(3.0 nm)/Tb(2.0 nm)] 20 multilayers along with their fit deposited on FG substrates with different etching times. The substrate roughness (ss) and interface roughness (si) are shown. The inset shows the transverse (o) scan for S1 along with 55 the fit at two different angles of y corresponding to the position at the Bragg peak and at an off-set to it. At o2y the specular peak is 111 seen over a diffuse background. For clarity, various curves are shifted relative to each other along the y-axis. MAGMA : 8449 ARTICLE IN PRESS A. Paul / Journal of Magnetism and Magnetic Materials ] (]]]]) ]]]­]]] 3 1 due to the correlated part of the roughness [8] only. The 57 fit to the rocking curve (inset for S1) shows the 4.0 3 correlation length for the correlated and uncorrelated 59 part of s as x 3.5 cB300 nm and xuB50 nm with h ¼ 5 0:570:02 and do not change with sU The polycrystalline 61 3.0 nature of the Fe layer in the Fe/Tb MLs is confirmed for GMR (%) 7 all the specimens from the XRD measurements. 2.5 63 The angle f (73.01) (between the film normal and the 3.0 9 average direction of the magnetic moments) as obtained 2.0 65 2.5 from the fit of the room temperature CEMS spectra of 11 four representative samples S1, S4, S5, and S8 is B421 2.0 67 [9]. A small decrease in PMA though may be observed 13 for sample no. S8 (fB541) whose roughness is compar- Substrate 1.5 69 roughness (nm) able with the thickness of the layer. It may be noted 15 from an earlier study [1] that a small change in s 1.0 71 (B0.45 nm) by 80 MeV Si ion-irradiation causes the 0 200 400 600 800 17 angle f to decrease by 6.31, whereas in the present case Time of etching (s) 73 the roughness has been increased by 2.5 nm resulting in a Fig. 2. The plot of surface roughness of the FG substrates and 19 similar change in f: The intermixed layer thickness has the corresponding GMR ratio as obtained from the fit to the 75 also been found to increase (upon 150 MeV Ag ion- XRR data and the magnetoresistance measured. The arrows 21 irradiation [1]) or decrease (by thermal annealing [3]) indicate the points for variation maximum/minimum in rough- 77 causing the PMA to decrease largely. Therefore, in both ness/GMR. 23 post-deposition treatments where the s variation is 79 expected to be uncorrelated is seen to be associated with 0.70 25 a possible stress relaxation within the bulk of the layers 81 causing the PMA to decrease but a correlated change in 27 roughness does not affect the PMA significantly. 6 ) 0.65 s 83 /M r 29 3.2. Fe/Cr MLs ) -M s 85 /M (1 r 0.60 5 ) / 31 In Set2 the substrate roughness variation, the s and x -M(1 (% 87 (from XRR and XDS) and the intermixed layer RM 33 thickness (from CEMS) of the MLs are found to behave 0.55 G 89 similarly as in Set1. XRD measurements have shown 4 35 that the structural coherence length (z), grain size, 0.50 91 internal stresses and the texture (1 1 0) do not vary from 1.0 1.5 2.0 2.5 3.0 37 sample to sample. Furthermore, since all the films were Substrate roughness (nm) 93 deposited simultaneously, the deposition conditions like Fig. 3. The change in AFF (') with increase in substrate 39 deposition rate and substrate temperature are identical roughness as obtained from SQUID measurements. Also 95 for all the specimens; therefore, the individual layer shown is the GMR normalised to AFF (") with increasing 41 thicknesses as well as the density of defects in the bulk of roughness. 97 the layers is expected to be similar. 43 Thus, the only difference between various MLs in case of PMA in Fe/Tb MLs). The uncorrelated 99 deposited on different substrates is in their s; and the roughness in all the MLs is expected to be similar in 45 observed variation in GMR can solely be attributed to magnitude because of the identical conditions of 101 the variation in the s: The GMR ratio is defined as deposition. The AFF showing a saturating behaviour 47 ðR0 RsÞ=Rs 100ð%Þ; with R0 and Rs being, respec- with increase in roughness is plotted in Fig. 3. Normal- 103 tively, the resistance values at zero and saturating fields. ising the GMR (%) ratio with AFF gives the contribu- 49 It is 105 51 107 53 UNCORRECTED PROOF interesting to note that with increase in etching time tion due to the interfacial scattering with the increase in as shown in Fig. 2, the variation in GMR is highly roughness, which is also plotted. One may see from the correlated with that in the roughness. The difference in figure that while the decrease in the AFF is B20%, the the interfacial roughness in different MLs is essentially interfacial scattering alone can bring B40% decrease in due to the difference in the roughness of their substrate GMR ratio for a change of B70% in correlated 109 which is transmitted to the successive layers. Thus, the roughness in a range of few nm. This change in GMR 55 difference among various MLs is expected to be in their is smaller as compared to B65% decrease due to 111 correlated part of the interfacial roughness (similarly as 200 MeV Ag ion irradiation effects as observed in an MAGMA : 8449 ARTICLE IN PRESS 4 A. Paul / Journal of Magnetism and Magnetic Materials ] (]]]]) ]]]­]]] 1 earlier study [10]. The interface structure modification References due to ion irradiation effects are expected to be 25 3 uncorrelated and thus a small increase in roughness [1] A. Gupta, R. Amitesh Paul, D.K. Gupta, G. Avasthi, can cause a large decrease in GMR. Principi, J. Phys. Condes. Mater. 10 (1998) 9669 and 27 5 In conclusion it has been seen that keeping all other references therein. parameters unchanged a large change only in the [2] A. Gupta, Amitesh Paul, S.M. Chaudhari, D.M. Phase, 29 7 correlated part of the interface roughness can be caused J. Phys. Soc. Jpn. 69 (2000) 2182 and references therein. by etching the substrates for different periods of time. [3] Amitesh Paul, A.Gupta, J. Alloys Compounds 2001, in press. 31 9 This correlated variation is expected to have a smaller [4] D.E. Savage, J. Kleiner, N. Schimke, Y.H. Phang, T. effect on the RE­TM bonds at the interface of Fe/Tb Jankowski, J. Jacobs, R. Kariotis, M.G. Lagalley, J. Appl. 33 11 systems or on the interfacial scattering in Fe/Cr MLs Phys. 69 (1991) 1411.D.K.G. de Boer, Phys. Rev. B 49 compared to the uncorrelated changes at the interfaces (1994) 5817. 35 13 caused by other post-deposition treatments as ion [5] R.A. Brand, Nucl. Instrum. and Methods B 28 (1987) 398. irradiation, thermal annealing and in situ modification [6] R. Schad, P. Beli.en, G. Verbanck, V.V. Moshchalkov, Y. 37 15 of roughness. Thus a small decrease in PMA and in Bruynseraede, H. Fisher, S. Lefebvre, M. Bessiere, Phys. GMR is observed with increase in roughness. Rev. B 59 (1999) 1242. 39 [7] L.G. Parratt, Phys. Rev. (1954) 359. 17 [8] A. Gupta, Amitesh Paul, S.Mukhopadhyay, Ko Mibu, J. Appl. Phys. 2001, in press. 41 19 Acknowledgements [9] Amitesh Paul, Ajay Gupta, Prasanna Shah, K. Kawaguchi, Hyperfine Interaction 2001, in press. 43 21 The work was done at Inter-University Consortium [10] A. Amitesh Paul, S.M. Gupta, D.M. Chaudhari, Phase, for DAEF (Indore), India. Vacuum 60 (2001) 401. 45 23 UNCORRECTED PROOF